This invention relates to zeolites, and especially zeolite A and mixtures of zeolite A and zeolite X having a small crystal size and particle size, and enhanced liquid carrying capacity, cation exchange rate, and cation exchange capacity. Such zeolites have a number of uses, but are especially useful as builders in combination with detergents in cleaning formulations.
Zeolites, as is commonly known in the art, are crystalline aluminosilicates having fully cross-linked open framework structures made up of corner-sharing SiO4 and AlO4 tetrahedral groups. Zeolites belong to the class of minerals referred to generally as tectosilicates, because their crystalline architecture can be idealized as being constructed from silicon atoms in tetrahedral, four-fold coordination with oxygen atoms in a 3-dimensional lattice. Each silicon atom in the structure has a nominal 4+ charge and shares 4 oxygen atoms (each having a nominal charge of 2xe2x88x92) with other silicon atoms in the crystal lattice.
Substitution of the isoelectronic Al3+xe2x88x92 for Si4+ creates a charge inbalance on the lattice that must be rectified by the incorporation of additional cations close by Al sites in the framework. Steric accommodation of these cations directs the crystallization of aluminosilicates towards the formation of more open structures containing continuous channels or micropores within the crystal. These structural micropores in the anhydrous zeolites allow the passage and adsorption of molecules based on size giving the materials molecular sieving properties. The cations themselves are not part of the crystal framework and can usually be replaced by equivalently charged species without damage to the lattice. In zeolite structures such as A and X the pore size is large enough to permit the facile passage and exchange of cations in aqueous solutions. The as-synthesized forms of zeolites A and X contain Na cations that can be exchanged for Ca2+ and Mg2+ ions present in so-called xe2x80x9chardxe2x80x9d waters and this gives these two zeolites particular value as water xe2x80x9csofteningxe2x80x9d builders in detergent formulations.
Zeolites in general can be represented empirically as:
M2/nO.Al2O3.xSiO2.yH2O;
wherein: M represents an exchangeable cation of valence n which is 1 or 2; x represents the number of moles of silica per mole of alumina and is typically about 2 for NaA and 2-3 for zeolite X; and y represents the number of moles of water per mole of alumina. M is typically a Group I or II ion, although other metal, non-metal and organic cations may also balance the negative charge created by the presence of aluminum in the structure. In addition to Si4+ and Al3+, other elements can also be present in the zeolitic framework.
Zeolites are frequently categorized by their crystalline structure. See W. M. Meier, D. H. Olson, and C. Baerlocher, Atlas of Zeolite Structure Types, Elsevier Press (1996) 4th edition. Among these structure types are zeolite A and zeolite X, which are the subject of the present invention. Zeolite A has the usual formula of: Na2O.Al2O3.2.0SiO2.4.5H2O, and zeolite X has an empirical formula of: Na2O.Al2O3.xSiO2.6H2O, wherein x is in the range of 2-3.
The microporous structure makes zeolites useful in a number of industrial applications, such as drying agents molecular sieves (highly selective adsorbents), ion exchangers, and catalysts. Particles consisting of agglomerated zeolite crystals also have a macroporosity that is useful in the manufacture of dry laundry detergents, for example, where the particles act as a carrier for liquid detergent chemicals. The amount of liquid detergent chemical that can be carried by a particular zeolite powder is indicated by its liquid carrying capacity (LCC), often expressed as the grams liquid per 100 grams of xe2x80x9cas-isxe2x80x9d zeolite. Unless otherwise indicated herein, the xe2x80x9cas isxe2x80x9d weight of the zeolite includes any interstitial water of hydration. Zeolites for application as detergent builders are typically sold in a hydrated form wherein the weight of the hydrated zeolite is approximately 20-22% water, also referred to as 20-22% LOI. LOI stands for the xe2x80x9closs on ignitionxe2x80x9d resulting when a zeolite sample is heated to a specified elevated temperature to drive off volatile components such as water or organic materials.
The effectiveness of a detergent is often influenced in complex ways by the xe2x80x9chardnessxe2x80x9d of the water. Water hardness is measured in terms of the weight of CaCO3 (in parts per million, ppm) equivalent to the concentration of soluble Ca and Mg present in water. Ca and Mg cations interfere with the action of the detergent in removing dirt from articles of clothing by reacting with detergent species. Ca in the dirt itself is thought to promote adhesion to fabrics and extraction of Ca by the zeolite may amplify the effectiveness of the detergent.
The Na-form of zeolite A exhibits a highly selective exchange affinity for Ca2+ ions, the primary cation found in potable water in the United States, whereas zeolite X has a particularly high affinity for exchanging both calcium and magnesium ions. The greater facility with which the X phase takes up Mg2+ is believed to be due to the larger pore size of this zeolite which more readily accommodates entry of the significantly larger hydrated Mg cation. When Mg is present in solution in high proportion it also interferes and slows the rate of Ca uptake by zeolite A. For waters containing predominantly Ca, zeolite A alone provides satisfactory exchange performance, but for waters containing higher proportions of Mg as well, it is advantageous to use combinations of zeolites A and X. In such applications it is preferable to use a zeolite X component of the so-called xe2x80x9clow silicaxe2x80x9d variety (LSX) with a composition and exchange capacity per unit weight that is equivalent, or nearly so, to that of zeolite A. The separate manufacture of LSX for use in combination with zeolite A is more expensive, so it is advantageous to accomplish direct synthesis of the mixed zeolite Group I ion product in the same low cost process used to manufacture zeolite A.
To maximize the effectiveness of detergent components of a washing formulation, it is critically important to remove the hardness components from the wash water as rapidly as possible. Ca removal, or sequestration, by a solid material occurs via a sequence of steps: a) Ca2+ diffusion through the solution to the zeolite particle; b) diffusion of Ca2+ across the static film boundary at the crystal/solution interface; c) distribution of Ca2+ over exchange sites by diffusion of the ion through zeolite micropores. The slowest, and therefore rate-determining, steps in this exchange process are believed to be associated with diffusion across the film boundary layer and distribution through the crystal. Vigorous agitation in the solution phase and dispersion of zeolite powder in the liquid facilitates transfer of Ca2+ through the bulk solution so that this step is not rate-limiting. Recognizing this, zeolite manufacturers make every effort to manufacture zeolite detergent builder materials with smaller particle size so as to increase the net rate of Ca/Mg sequestration. Larger particles, especially those greater than 10 microns in diameter, must also be minimized to avoid the unsightly deposition of zeolite residues on dark colored articles of clothing. As would be expected, the rate of Ca removal from solution is strongly dependent upon the temperature of the exchange solution. Ca diffusion processes, and hence their exchange rates, in zeolites occur more rapidly in hot water than in cold. To be useful as performance builders in detergent powders for cold water application, it is desirable to improve the exchange rate of Ca on zeolite A powders.
One strategy to increase exchange rate is to significantly reduce zeolite particle size. For detergent zeolite powder, however, other criteria constrain the manufacture of very small zeolite particles. Cost is an important market consideration. Some methods for the synthesis of very small particles require more dilute synthesis gels with reduced batch yield, and this raises unit costs of manufacture. Other methods require more costly reagents or more eccentric compositions that complicate the recycle operations typically of efficient, low-cost manufacturing processes. Ultra-small particles become much more difficult to separate from is their synthesis mother liquors and wash effectively without the use of flocculating agents. The use of flocculating agents to facilitate solids separation is undesirable due to the potential for interaction of such agents with components of detergent formulations and to the unfavorable effect of flocculents on the dispersibility of zeolite powders in wash waters. Very fine powders tend to have very low bulk densities that require expanded storage volumes and make for more difficult powder handling.
One performance measure used for zeolite A detergent builders is to measure the Ca2+ removed by a 0.5 gram sample of zeolite powder in a short time interval (2 minutes) from a solution at a fixed temperature containing 1000 ppm equivalents of CaCO3 as CaCl2. This quantity expressed as milligrams CaCO3 per gram anhydrous zeolite is taken as a measure of the calcium exchange xe2x80x9cratexe2x80x9d (CER). A second convention measures the quantity of Ca removed from the same solution after 15 minutes and equates this value to the effective Ca exchange xe2x80x9ccapacityxe2x80x9d (CEC) expressed in the same units. 15 minutes is considered a practical time for the zeolite exchange to have come very near its equilibrium limit. As referred to herein, CER and CEC are measured at a solution temperature of 10xc2x0 C. for applicability to cold water detergent applications. An exemplary CER and CEC procedure is described further herein. Obviously, good exchange performance in cold water invariably results in even better performance in water at higher temperature.
Zeolite A of this invention has a calculated maximum capacity of 7 milliequivalents per gram. This corresponds to a maximum theoretical exchange capacity equivalent to 350 ppm CaCO3. Under the conditions used to evaluate the Ca exchange capacity of the zeolite A of this invention the observed Ca exchange capacity is generally somewhat superior to the capacity of commercial VALFOR(copyright) 100. CEC values for the zeolite A of this invention fall in the range 280-300 when measured at 10xc2x0 C. whereas ordinary VALFOR(copyright) materials at the same temperature have CEC values of 250-280. The zeolite A of this invention is most distinguished from conventionally manufactured VALFOR(copyright) 100 in its more critical performance at 2 minutes in cold water where typical CER values are  greater than 200 and even  greater than 250 mg CaCO3 per g zeolite approaching equilibrium limit (CEC) values so that a much larger fraction of the total exchange capacity is put to use during the critical initial minutes of the washing cycle where effective Ca removal is most important to the function of the detergent.
The superior cold water Ca exchange performance of the zeolites of this invention can be related to the significantly increased external surface area of the zeolite crystals. The enhanced surface areas measured on the products can be converted by calculation into an equivalent diameter of uniformly sized spherical particles of the same intrinsic density that have the same specific surface area. For zeolite A of conventional manufacture this hypothetical diameter is comparable to the observed particle and crystal sizes of the actual material. For the products of this invention, however, the hypothetical equivalent spherical diameter is very much smaller than the median particle size of crystalline agglomerates and smaller even than the size of individual crystallites. This surface area enhancement is thought to be due to the dramatically increased roughness, or fractal property, of the crystal surfaces generated by important modifications of the procedures for zeolite A synthesis. These procedures are thought to promote more rapid and disorderly growth of zeolite A crystals that results in this fractal character.
Zeolites can be made by a number of processes. One generalized industrial process for making zeolite A is shown schematically in FIG. 1. The process shown in FIG. 1 first comprises preparing a sodium aluminate solution in digester 10. A soluble aluminate solution may be prepared by dissolving Al2O3.3H2O, also known as alumina trihydrate (ATH) or Al(OH)3, in a solution of NaOH and water. The composition of this solution can be varied over a considerable range in terms of weight percent Na2O or Al2O3 dissolved. Compositions that provide stable solutions at ambient temperatures can be described in a binary phase diagram well-known in the art, such as for example as published in the Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, Vol. 2, p. 269 (1992). The aluminate solution can be prepared and analyzed and stored for later use or it can be prepared to a specific formulation as a batch for each batch synthesis of zeolite. Hot aluminate solutions with temperatures up to approximately 200xc2x0 F. can also be used. In a commercial-scale operation, the sodium aluminate solution may be prepared by combining recycled xe2x80x9cmother liquorxe2x80x9d (filtrate 75 from filter 70), which contains caustic with small amounts of dissolved alumina. The Na-aluminate solution can have a concentration of anywhere from 5%-22% Al2O3. The slurry of water or mother liquor and ATH is typically heated for 15-20 minutes to dissolve some of the ATH powder. In some commercial-scale processes, some fraction of the alumina (for example, approximately 15-20%) may remain undissolved in this solution as alumina trihydrate. Where some fraction remains undissolved, the sodium aluminate material may be more properly termed a mixture; if all the alumina trihydrate dissolves, the mixture is referred to as a solution. As used herein, the term xe2x80x9cmixturexe2x80x9d refers to both solutions where all the soluble alumina is dissolved and mixtures where some undissolved alumina remains. The term sodium silicate mixture is also used herein to refer to solutions and mixtures.
When aluminate is prepared in bulk, it is frequently the practice to meter the requisite quantity of analyzed solution into a batch feed tank 20. A silicate batch feed tank 30 can similarly be supplied with a requisite quantity of soluble silicate solution of a specific composition. Alternatively, the soluble silicate of known composition can be supplied by a larger silicate storage tank (not shown). Soluble silicate for this purpose can be prepared by the dissolution of sodium silicate glasses in water or dilute solutions of NaOH by procedures well known in the art. Such silicate solutions can also be purchased from commercial suppliers. The sodium silicate solution may be mixed in a ratio of between 1.0-3.8 SiO2/Na2O at 150xc2x0 F.
The sodium aluminate mixture and sodium silicate solution may then be pumped directly into crystallizer 60, or optionally into gel mix tank 40 (described below). The sodium aluminate and sodium silicate streams may be fed sequentially or simultaneously into the mixing vessel through dedicated lines, or the streams may be mixed in-line, such as with a jet mixer, prior to being fed into the mixing vessel. The combination of soluble silicate and aluminate solutions typically generates an amorphous gel-like precipitate such that the slurry viscosity increases substantially. This viscosity diminishes over time and with heating. The high viscosity requires powerful agitator motors to effect mixing of the reagents. Thus, a separate mix tank 40 with powerful agitiation is sometimes supplied for the purpose of overcoming this initial high viscosity. Alternatively, the reagents can be mixed directly in crystallizer 60. Crystallizer 60 typically is equipped with baffles and agitators. Agitation and mixing of the ingredients is typically effected by means of one or more turbine paddles with pitched blades, and the tank itself is equipped with baffles to promote a high degree of turbulence.
Optionally, the gel can be held or xe2x80x9cagedxe2x80x9d in an aging tank 50 between gel mix tank 40 and crystallizer 60 for some period of time up to several hours at temperatures below the normal temperature of crystal growth to promote greater homogeneity and the formation of zeolite nucleii or precursor species. This treatment is believed to promote diffusive blending of the reactive ingredients and is known in the art to favor the formation of larger numbers of zeolite nucleii or precursor species in the gel mixture. Increased numbers of crystal nucleii generate smaller crystals in the product. An alternative is to provide small amounts of a xe2x80x9cseed gelxe2x80x9d containing nucleii or pre-cursor species that are able to generate nucleii in sufficient numbers to effect a significant reduction in crystal size of the product. One such xe2x80x9cseed gelxe2x80x9d is disclosed in U.S. Pat. No. 3,808,326 as a xe2x80x9cprecursor mixturexe2x80x9d for adding to synthesis compositions designed to produce the faujasite type zeolites, X and Y.
Crystallization of the gel mixture is carried out by a conventional process in which the gel is heated at temperatures between 80-100xc2x0 C. for some period of time until the slurry solids are fully crystalline as determined by comparison of a carefully made X-ray diffraction (XRD) scan with a reference scan of material known to be fully crystalline. The time required for crystallization is strongly dependent upon temperature with higher temperatures favoring more rapid crystallization. For equivalent formulations, a lower temperature favors crystallization of smaller crystals and particles over longer times while higher temperatures favor faster crystallization of larger crystals and particles.
After the crystallization step, a flash step may be performed to cool the mixture prior to filtration. Such a flash step may comprise placing the mixture under vacuum to flash off water vapor, thus cooling the mixture. The flashed-off water vapor may then be condensed and used as de-ionized water in the subsequent filtration step.
The crystalline product is separated from the mother liquor by filtration using various standard filtration processes and equipment including belt filters or filter presses 70. The product is washed with an appropriate amount of water, such as the de-ionized water created in the flash step described above, to displace residual mother liquor so as to give consistent properties meeting specifications for the dried product. The filtrate liquid 75 consisting of the crystallization mother liquor contains un-utilized reagent values in the form of soluble Na2O and Al2O3 or SiO2. These values can be recovered by recycle of the mother liquor as is or after appropriate evaporation of excess H2O so as to maintain water balance in the manufacturing process. In the case of crystallization of zeolite A (with SiO2/Al2O3=2.0) synthesis formulations having gel SiO2/Al2O3 mole ratios less than 2.0 invariably generate a mother liquor which contains excess Al2O3 rather than SiO2.
Filter cake 77, which comprises approximately 60-65% solids, is dried in a dryer 80 by conventional methods for fine powder drying to an LOI in the range 18-24 weight percent. Conventional drying methods include various continuous methods including flash drying or spray drying as well as batch drying in ovens. For example, a ring dryer run with a 190xc2x0 F. outlet temperature may achieve the desired result. The zeolite powder may then be captured in standard solid/gas separation equipment known in the art, such as a baghouse (not shown). The zeolite powder is then typically transferred to storage silos 90 from which the product can be transferred to, for example, bags, supersacks, trucks, or railcars.
The bulk chemical reaction for making zeolite A by this exemplary batch process essentially comprises mixing together alkaline aqueous solutions of sodium silicate and sodium aluminate to form amorphous aluminosilicate gels which are heated for a time and converted to a crystalline aluminosilicate with an ideal anhydrous oxide composition of: 2SiO2.Al2O3.Na2O, having a characteristic x-ray diffraction pattern and commonly referred to as sodium zeolite A, zeolite A, 4A, or LTA.
It is common in zeolite A synthesis, though not essential, to use a batch formulation containing less than the stoichiometric amount of SiO2 required for the product composition. In such formulations, SiO2 is the limiting reagent so that the separation of product solids leaves the solution phase mother liquor containing some small amount of dissolved Al2O3 together with significant concentrations of Na2O and negligible (ppm) amounts of soluble SiO2.While ordinary synthesis procedures generate a product that has an analytical composition very close to the ideal stoichiometry, compositions can also be synthesized that have both higher and lower mole ratios of SiO2/Al2O3 and Na2O/Al2O3, detected by careful chemical analysis, which yet exhibit essentially the same x-ray diffraction pattern. Such composition variations can be obtained through either inadequate or excessive washing of the product to free it of retained synthesis liquors, or due to genuine differences arising from the incorporation of more SiO2 in the crystal framework or the occlusion of Na-silicate or Na-aluminate species within the crystal structure of ideal composition.
In conventional batch methods of Na-A synthesis, combination of Na-silicate and Na-aluminate solutions results in the formation of a highly viscous gel phase. This gel viscosity increases with solids content in the slurry so that solids content, and hence batch yield, is subject to practical limitations due to the exponential increase in cost for agitation equipment that will provide the requisite degree of mixing needed to get good heat transfer and combination of reactants. Forming less-viscous gels allows higher solids formulations to be used, increasing yields in the same equipment and reducing the unit cost of manufacture accordingly. Other methods of increasing yields or reducing batch cycle time also contribute to lower unit manufacturing costs. In batch manufacturing processes, a targeted increase in the scale of production typically requires a linear increase in capital equipment, including building space to house that equipment.
It is therefore particularly advantageous and desirable to provide manufacturing procedures that allow significant expansion of batch yield using existing capital equipment, or which require manufacturing capacity expansion in only a part of the series of unit operations. The process improvements of this invention allow the implementation of such strategies to improve the yield of existing manufacturing facilities substantially with only minimal capital investment.
The present invention comprises zeolite products having improved properties as a result of novel modifications to the standard zeolite synthesis process. One embodiment of this invention comprises zeolite A with superior Ca ion exchange properties such that the zeolite can be used in cold water detergent washing formulations without a water softening performance penalty. A second embodiment of this invention comprises zeolite A and X mixtures that provide the enhanced water softening performance in the presence of high Mg. The X product in such A/X mixtures may be a low silica variety (sometimes referred to as LSX) that has nearly the same chemical composition and total ion exchange capacity as zeolite A. Thus, the use of the A/X mixtures of this invention incurs a negligible penalty in reduced total ion exchange capacity per unit weight of zeolite powder.
The synthesis of either the pure A or A/X mixtures of this invention can be accomplished in a commercial manufacturing process without changing existing reagents or using additional reagents and even without changing the proportions of oxide components used in the synthesis formulation. Thus, existing manufacturing facilities can supply a more varied product to meet customer-specific requirements without changes in materials inventory, synthesis equipment, or material balances that may affect recycle operations within the facility.
The superior exchange properties of the zeolite of this invention are evidenced most clearly in terms of their cold water (10xc2x0 C.) Ca exchange performance. Values obtained from the measurement of Ca exchange rate (CER) are dependent upon the conditions and method of measurement including composition and concentration in test solutions. This is partly true of measurements of Ca exchange capacity (CEC). For this reason it is best to compare the performance of the improved zeolite exchange product with a typical commercial product such as VALFOR(copyright) 100 (manufactured by the PQ Corporation of Valley Forge, Pa.) under precisely the same experimental conditions. When the products of this invention are compared with representative samples of VALFOR(copyright) 100 we find an improvement in the CER values in a range from 15 to 50 percent. Typical VALFOR(copyright) 100 gives 2-minute CER values at 10xc2x0 C. in the range 160-170 up to about 190 mg of CaCO3 per gram anhydrous zeolite in the best quality material. The zeolite A of this invention, measured under identical circumstances, has a 2-minute CER in cold (10xc2x0 C.) water of greater than about 200, preferably greater than 220 and most preferably equal to or greater than 250 mg CaCO3/g anhydrous zeolite. The calcium exchange capacity (CEC) at 15 minutes in cold water is greater than about 250, preferably greater than about 260 and even as high as 300, milligrams of CaCO3 per gram of anhydrous zeolite. The corresponding CEC value for VALFOR(copyright) 100 in cold water is on the order of 250 mg CaCO3 per gram anhydrous zeolite.
Zeolite powders of this invention have a median particle size in the 1-5 micron range and most preferably in the 2-4 micron size range. VALFOR(copyright) 100 typically has a somewhat larger mean particle size in the range 4-5. Crystal sizes of the zeolite of this invention are fairly uniform in a range from about 0.2-0.8 microns and most preferably around 0.5 microns or slightly less. Some useful control of both particle and crystal size can be exercised by small adjustments of synthesis conditions especially with respect to reducing the larger particle size fraction in the distribution. Crystal size distribution is conspicuously smaller and more narrow than the distribution of crystal sizes observable in VALFOR(copyright) 100 by electron microscope examination. Bulk density typically falls in a range from about 0.19 to 0.35 g/milliliter.
A remarkable and unexpected feature of the product of this invention is that the external surface area of the zeolite crystals is significantly greater than the external surface area measured on a commercially available detergent zeolite such as VALFOR(copyright) 100. Moreover, when a calculation is made of the equivalent spherical diameter (ESD) for uniformly sized particles of the same intrinsic density having the identical specific surface area (m2/g) we find that this dimension is plausibly close to the measured particle size of the commercially available zeolite, but that the ESD calculated for the product of this invention is very much smaller than both the measured median particle size and the individual crystal size of the product. This high surface area property appears to be a direct consequence of the process of making the zeolite of this invention. Furthermore, while it is expected from the teachings of prior art that Ca exchange rates should increase as zeolite particle size decreases, we have found a negligible correlation of exchange rate with either particle or crystal size, but have found instead that the 10xc2x0 C. CER of the zeolite A of this invention is directly related to the external zeolite crystal surface area as measured by nitrogen adsorption using the well known B-E-T method. Specifically, the invention comprises a zeolite product comprising zeolite A having an external surface area greater than about 5 m2/g as measured by a B-E-T nitrogen adsorption method at the boiling point of liquid nitrogen. The increased surface area found in the product of this invention is equivalent to the area that would be generated by particles of uniformly sub-micron size. Such small particles would be prohibitively expensive to manufacture by synthesis techniques known in the art because of requirements for more costly reagents, lower yields, and difficulty in solids separation, recovery, and washing. The particles of this invention, however, are greater than about 1.9 microns, and preferably greater than about 3 microns in size, avoiding the problems inherent in small particles.
The ability of powders to absorb liquids while retaining powder flow properties is important in the formulation of powder detergents where active ingredients are often liquid materials. Liquid carrying capacity (LCC) is the weight of test liquid that can be taken up by 100 grams of powder without causing agglomeration of the powder to form a paste. The materials of this invention have, as another consequence of their altered morphology, improved liquid carrying capacity (LCC) when compared to a more conventional product of current commerce. Materials with a liquid carrying capacity substantially greater than 50 and even up to 100 may be prepared. LCC values are conveniently expressed as grams of liquid taken up or xe2x80x9ccarriedxe2x80x9d per 100 grams of the as-is zeolite (hydration water included). This valuable property enhancement appears to be directly caused by, or at least strongly associated with, the same physical and morphological changes that appear to give the product of this invention enhanced ion exchange performance.
The present invention also provides a process for making the zeolite product. The process comprises mixing a sodium silicate solution, a sodium aluminate reagent mixture, and an amorphous aluminosilicate initiator or xe2x80x9cseedxe2x80x9d gel in a mixing vessel to create an aluminosilicate synthesis gel and crystallizing said synthesis gel to form zeolite crystals. The process comprises adding the sodium aluminate reagent mixture to the sodium silicate solution preferably at a gradual rate such that the addition of the full charge of aluminate reagent requires longer than 20 minutes. Agitation during the aluminate addition is preferably vigorous to ensure a high degree of turbulent blending. Longer addition times up to 60 minutes can be beneficial but at increasing penalty in terms of process cycle time. The sodium aluminate reagent mixture may consist of a fully dissolved solution of alumina in NaOH or, advantageously, a slurry mixture in which a percentage of the total batch alumina is present in the form of a soluble aluminum oxide or hydroxide powder such as the alumina trihydrate (gibbsite, hydragillite).
The amorphous aluminosilicate initiator gel may be added before, after, or during the addition of the sodium aluminate to the mixing vessel, including adding the initiator gel to the sodium silicate solution prior to mixing with aluminate. The weight of alumina (expressed as Al2O3) added in the amorphous aluminosilicate initiator (xe2x80x9cseedxe2x80x9d) gel preferably comprises about 0.01 to about 2.0 weight percent of the total batch alumina, and more preferably, about 0.1 to about 1 weight percent.
Instead of, or in addition to, the gradual addition of the sodium aluminate mixture, the process using an initiator gel may further advantageously comprise adding a percentage of the batch alumina as alumina trihydrate powder. When the percentage of the total batch alumina added as alumina trihydrate powder comprises less than a limit point (about 35 weight percent in one set of conditions), the resulting zeolite product comprises almost entirely zeolite A. When the percentage of the total batch alumina added as alumina trihydrate powder comprises greater than about 35 weight percent, the resulting zeolite product comprises a mixture of zeolite A and zeolite X. In general, under a single set of synthesis conditions, the greater the fraction of alumina added as alumina trihydrate powder, the higher the percentage of the zeolite product as zeolite X. Thus, a zeolite product having a wide range of ratios of zeolite A to zeolite X can be made by varying the amount of alumina added as alumina trihydrate powder in a given composition. It will be appreciated by those skilled in the art of zeolite synthesis that the particular proportions of A and X formed in the mixed phase product will also depend on other parameters of the synthesis and the proportions of other synthesis components even with a fixed fraction of undissolved alumina. Changes in solution alkalinity and agitation, shift the concentration and composition of solute species and the rates of dissolution for both the gel phase and the alumina solids. Typically, these factors must be evaluated empirically for particular process compositions and configurations. The process of this invention allows significant flexibility in any existing manufacturing process to produce, in the same equipment, either pure A zeolite or variable mixtures of zeolites A and X to meet the specific needs of customers with no change in the basic quantities and proportions of synthesis ingredients, but only by varying the relative proportion of dissolved and undissolved alumina in the formulation.
In the investigation of the process of this invention we have further discovered that a superior quality pure zeolite A phase can be made under conditions similar to those just described for the synthesis of A/X mixtures. When similar gel compositions containing variable, large, proportions of undissolved alumina are prepared without the addition of the initiator (xe2x80x9cseedxe2x80x9d) gel the synthesis product consists of a pure zeolite A phase having a uniform small particle size with a significantly more narrow size distribution. Compositions with greater than 22 weight percent, preferably greater than 30 weight percent, more preferably between about 35 to about 90 weight percent, and even more preferably between about 60 to about 90 weight percent of the batch alumina present in undissolved form produce zeolite A product in systems that generate A/X mixtures in the presence of an initiator gel.
The invention further comprises zeolite A or a mixture of zeolite A and zeolite X made by the above processes, and detergent compositions containing zeolites made by the above processes. Such detergent compositions comprise from 0.1% to 99% of a builder system comprising the zeolite product of this invention, and optionally, a auxiliary detergent builder salt, and from 0.1% to 99% of by weight of at least one detergent adjunct other than the builder system.
The invention further comprises a process for increasing manufacturing throughput of existing equipment in a zeolite production facility. The process comprises adding an excess amount of undissolved soluble alumina well above the amount required to react with the amount of silicate added in the initial gel batch make-up. In this process the excess alumina dissolves rapidly during and after completion of a first crystallization stage and generates a zeolite A slurry in an alumina enriched liquor. The subsequent addition of an appropriate stoichiometric amount of concentrated soluble silicate to this slurry results in the rapid generation of additional zeolite with essentially no increase in synthesis cycle time. We have found that it is beneficial not to supply the exact stoichiometric amount of silica to react with all the available alumina in the mother liquor composition, but instead to limit the amount of added silicate so as to form a zeolite A with the typical composition 2.0SiO2.Al2O3.Na2O, leaving sufficient dissolved Al2O3 in solution to give a mother liquor Na2O/Al2O3 mole ratio less than about 60 and preferably about 20 to about 40. At higher ratios it appears that the hot, alkaline, mother liquor solution attacks the zeolite causing damage and degradation of properties, or else promotes the dissolution of the zeolite, forming an aluminosilicate that precipitates amorphous material on the product as the zeolite mother liquor cools slowly during downstream processing.
The process of this invention provides significant flexibility for existing manufacturing facilities in that both performance qualities and material handling properties of zeolites useful for detergent applications can be controlled and optimized with minimal or no requirement for additional capital investment, change of overall composition, or the introduction of new and more costly reagents.
The products of this invention have the remarkable quality of exhibiting the ion exchange rate behavior expected of very fine particles, while at the same time they maintain a particle size distribution and material handling properties more characteristic of conventional zeolite powders. For the zeolites of this invention, we have found that the exchange rate is dependent not upon the particle or crystal size of the zeolites but is strongly correlated with the external surface area of the zeolite particles. Zeolites of this invention have exceptionally large external surface areas as compared with more conventional zeolites prepared by the methods of prior art. The high surface area of the zeolite A of this invention appears to be due to a combination of factors in the particular procedures used in their synthesis.